Control device for internal combustion engine

ABSTRACT

A switching mechanism switches the pressure stage of oil, which is fed from an oil pan to the individual parts of an internal combustion engine, between two stages, which are a low-pressure stage and a high-pressure stage. An electronic control device controls the action of the switching mechanism. The electronic control device determines that the liquid level of the oil is abnormal on the basis of the fact that the liquid level of the oil, which is detected by a liquid level sensor, in the oil pan becomes lower than a predetermined value. If the pressure stage of the oil is set at the low-pressure stage through the switching mechanism, the predetermined value is set at a higher value than that when the pressure stage is set at the high-pressure stage.

TECHNICAL FIELD

The present invention relates to a control device for an internal combustion engine.

BACKGROUND ART

Conventionally, a typical internal combustion engine is provided with an oil pan for storing oil and an oil pump driven by a crankshaft, which is an output shaft of the engine. The oil stored in the oil pan is drawn and pressurized by the oil pump and then is fed to individual parts of the engine that need oil. A relief valve is provided downstream of the oil pump. When the oil pressure applied to the relief valve in a valve opening direction reaches a predetermined valve opening pressure or greater, the relief valve is opened and some of the oil ejected from the oil pump is released to the upstream side of the oil pump. Such a control device for an internal combustion engine is described in, for example, Patent Document 1.

When the liquid level of oil in the oil pan is excessively low, air is drawn along with the oil, which lowers the oil pressure. As a result, seizure of the internal combustion engine can occur. Thus, in some control devices, a liquid level sensor detects the liquid level of the oil, and when the liquid level of the oil lowers and falls below a predetermined value, it is determined that the liquid level of the oil is abnormal and some measure such as lighting of an alarm light is taken.

A device in which the pressure stage of oil fed to individual parts of an internal combustion engine is switched between two stages, a low-pressure stage and a high-pressure stage, by varying a valve opening pressure of the relief valve has been developed. In such a control device for an internal combustion engine, for example, when there is no need to significantly increase the pressure of the oil fed to individual parts of the engine, the mechanical load of the engine for driving the oil pump can be reduced by setting the pressure stage of the oil to the low-pressure stage, which improves fuel efficiency.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-107485

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

However, if the above-mentioned conventional determination of the liquid level of the oil is applied to the control device for an internal combustion engine that switches the pressure stage of the oil between a plurality of stages, the following problems occur. That is, in the case where the pressure stage of the oil is set to the low-pressure stage, the amount of the oil released to the upstream side of the oil pump increases as compared to the case where the pressure stage of the oil is set to the high-pressure stage. Accordingly, the amount of the oil drawn from the oil pan decreases. For this reason, the liquid level of the oil in the oil pan rises. However, in the above-mentioned liquid level determination, a predetermined value used in comparison with the liquid level of the oil is set to a common value irrespective of the pressure stage of the oil. For example, the predetermined value in the above-mentioned determination may be set to a relatively large value on the basis of the case where the pressure stage of the oil is set to the low-pressure stage. In this case, when the pressure stage of the oil is set to the high-pressure stage and the liquid level of the oil lowers, a false determination that the liquid level is abnormal may be made despite that the liquid level of the oil is normal. In contrast, the predetermined value in the above-mentioned determination is set to a relatively small value on the basis of the case where the pressure stage of the oil is set to the high-pressure stage. In this case, while the pressure stage of the oil is kept to the low-pressure stage, the phenomenon cannot be recognized unless the liquid level falls below the predetermined value, even if the liquid level of the oil excessively lowers. Thus, when the pressure stage of the oil is switched from the low-pressure stage to the high-pressure stage, the liquid level of the oil greatly falls below the predetermined value, with the result that suction of air can occur, which may cause seizure of the internal combustion engine.

These problems do not occur only in a device that switches the pressure stage of the oil between a plurality of stages by adjusting the valve opening pressure of a relief valve, but commonly occur in control devices for an internal combustion engine that control a switching mechanism between the plurality of stages for switching the pressure stage of oil fed from a storage portion to individual parts of the internal combustion engine.

An objective of the present invention is to provide a control device for an internal combustion engine including a switching mechanism for switching the pressure stage of oil between a plurality of stages, and the device can correctly determine abnormality of the liquid level of the oil.

Means for Solving the Problems

To achieve the foregoing objective and in accordance with one aspect of the present invention, a control device for an internal combustion engine is provided. The engine includes a switching mechanism for switching a pressure stage of oil, which is fed from a storage portion to individual parts of the engine, between a plurality of stages. The control device includes a control portion for controlling action of the switching mechanism, a liquid level sensor for detecting the liquid level of oil in the storage portion, a determination portion, and a setting portion. The determination portion determines that the liquid level of the oil is abnormal based on the fact that the oil liquid level detected by the liquid level sensor falls below a predetermined value. The setting portion sets the predetermined value to a larger value if the pressure stage of the oil is set to a low-pressure stage through the switching mechanism, as compared to a case where the pressure stage of the oil is set to a high-pressure stage.

In the case where the pressure stage of the oil is set to the low-pressure stage, the liquid level of the oil in the storage portion becomes higher compared to the case where the pressure stage of the oil is set to the high-pressure stage. With the above-mentioned configuration, the predetermined value used in comparison with the liquid level of the oil in the liquid level determination is not set to a common value irrespective of the pressure stage of the oil, but is variably set on the basis of the above-mentioned relationship between the pressure stage of the oil and the liquid level of the oil. Therefore, in the internal combustion engine that includes the switching mechanism for switching the pressure stage of the oil between the plurality of stages, abnormality of the liquid level of the oil can be correctly determined.

It is preferable that the determination portion prohibit determination until a predetermined period has passed after the control portion switches the pressure stage of the oil.

It takes a predetermined time until, after switching of the pressure stage of the oil, the liquid level of the oil in the storage portion reaches a level corresponding to the switched pressure stage. For this reason, for example, when the liquid level of the oil is determined on the basis of the predetermined value set according to the switched pressure stage immediately after the pressure stage of the oil is switched, a false determination can be made.

According to the above-mentioned aspect, after switching of the pressure stage of the oil, the liquid level of the oil is prevented from being determined before the liquid level of the oil in the storage portion reaches the level corresponding to the switched pressure stage. Therefore, it is possible to prevent a false determination from being made due to the above-mentioned determination made immediately after switching of the pressure stage of the oil.

It is preferable that the determination portion determine that the oil liquid level is abnormal based on the fact that the oil liquid level detected by the liquid level sensor remains below the predetermined value for a predetermined period.

It takes a predetermined time until, after switching of the pressure stage of the oil, the liquid level of the oil in the storage portion becomes a level corresponding to the switched pressure stage. For this reason, for example, when the liquid level of the oil is determined on the basis of the predetermined value set according to the switched pressure stage immediately after the pressure stage of the oil is switched, false determination can be made.

With the above-mentioned configuration, after switching of the pressure stage of the oil, even when the liquid level of the oil temporarily falls below the predetermined value before the liquid level of the oil in the storage portion becomes the level corresponding to the switched pressure stage, the liquid level of the oil is not determined as abnormal if the duration of such state is less than a predetermined period. Therefore, it is possible to prevent a false determination from being made due the drop of the liquid level of the oil below the predetermined value immediately after switching of the pressure stage of the oil.

In accordance with another aspect of the present invention, the switching mechanism comprises a relief valve for releasing some of oil ejected from an oil pump driven by the internal combustion engine to an upstream side of the oil pump and a change portion for changing a valve opening pressure of the relief valve.

In accordance with a further aspect of the present invention, the switching mechanism switches the pressure stage of the oil between two stages, which are a low-pressure stage and a high-pressure stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a control device for an internal combustion engine in accordance with one embodiment of the present invention;

FIG. 2( a) is a cross-sectional view showing the relief valve in FIG. 1 in a state where a movable member is located at a first position and a valve body is in the closed state;

FIG. 2( b) is a cross-sectional view showing the relief valve in a state where the movable member is located at the first position and the valve body is in the opened state;

FIG. 3( a) is a cross-sectional view showing the relief valve in FIG. 1 in a state where the movable member is located at a second position and the valve body is in the closed state;

FIG. 3( b) is a cross-sectional view showing the relief valve in a state where the movable member is located at the second position and the valve body is in the opened state;

FIG. 4 is a flowchart showing a processing procedure of energization control of a switching valve;

FIG. 5 is a map showing the relationship among engine speed, engine load and oil pressure stage;

FIG. 6 is a graph for describing changes of oil pressure with changes in the engine speed;

FIG. 7 is a flowchart showing a processing procedure of liquid level determination; and

FIG. 8 is a map showing relationship between the engine speed, and a high-pressure stage predetermined value and a low-pressure stage predetermined value.

MODE FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1 to 8, one embodiment of a control device for an internal combustion engine according to the present invention will be described below.

FIG. 1 shows schematic configuration of the control device for an internal combustion engine in accordance with this embodiment.

As shown in this drawing, the internal combustion engine is provided with a main supply passage 11 for feeding oil stored in an oil pan 12, or storage portion, to individual parts of the internal combustion engine. An engine-driven oil pump 14 is provided at a midpoint of the main supply passage 11. An oil strainer 13 is provided at an upstream end of the main supply passage 11 (an end at the oil pan 12). An oil filter 15 is provided downstream of the oil pump 14 in the main supply passage 11. When the oil pump 14 is driven, the oil in the oil pan 12 is drawn and pressurized by the oil pump 14 through the main supply passage 11 and is ejected to a downstream side of the passage 11. The oil thus ejected is fed to the individual parts of the internal combustion engine, such as various hydraulically-driven devices driven by the pressure of the oil, a piston jet mechanism for jetting the oil to a piston for taking an engine output to cool the pistons and parts of the internal combustion engine that need lubrication (neither is shown).

The internal combustion engine is provided with a switching mechanism 20 for switching the pressure stage of the oil, which is fed to the individual parts of the engine, between two stages, a high-pressure stage and a low-pressure stage. The switching mechanism 20 includes a relief passage 16 provided at the main supply passage 11, a relief valve 21 provided at a midpoint of the relief passage 16, an introducing passage 28 provided at the relief passage 16, and a switching valve 29 provided at a midpoint of the introducing passage 28.

The relief passage 16 is connected to a downstream side of the oil filter 15 in the main supply passage 11 at one end and is connected to an upstream side of the oil pump 14 and a downstream side of the oil strainer 13 in the main supply passage 11 at the other end. The relief valve 21 is provided in the relief passage 16. When a force based on the pressure of the oil ejected from the oil pump 14 reaches a predetermined valve opening pressure or larger, the relief valve 21 opens and releases some of the oil ejected from the pump 14 to the upstream side of the pump.

The relief valve 21 includes a cylindrical housing 22, which is closed at one end and is provided at a midpoint of the relief passage 16, and a cylindrical valve body 25, which is accommodated in an accommodation chamber 23 as an internal space of the housing 22. The cylindrical valve body 25 is displaceable in an axial direction of the housing 22 (hereinafter referred to as axial direction A). The accommodation chamber 23 has a cylindrical movable member 24, which is closed at one end and is displaceable in the axial direction A. The housing 22 and the movable member 24 are provided so that their bottom portions 22A, 24A are located upstream in the relief passage 16 (upper side in FIG. 1). The relief valve 21 includes a fixation member 26 that covers an opening an end 225 of the housing 22, and a spring 27, which is provided between the valve body 25 and the fixation member 26 and biases the valve body 25 toward the bottom portion 24A of the movable member 24. The fixation member 26 has a cylindrical large diameter portion 26A and a cylindrical small diameter portion 268, which is provided integrally and coaxially with the large diameter portion 26A and has a smaller diameter than the large diameter portion 26A. An upper end surface of the large diameter portion 26A is in contact with a lower end surface of the end 22B of the housing 22, and a peripheral surface of the small diameter portion 26B is in contact with an inner peripheral surface of the movable member 24.

An inlet-side through hole 22C and an inlet-side communication hole 24C having the same diameter as the inlet-side through hole 22C are provided at the center of the bottom portion 22A of the housing 22 and the center of the bottom portion 24A of the movable member 24, respectively. The through hole 22C and the communication hole 24C form a portion of the relief passage 16.

An outlet-side through hole 220 that extends through a side portion of the housing 22 is formed substantially at a center position (left side in FIG. 1) in the axial direction A of the side portion. An outlet-side communication hole 24D that extends through a side portion of the movable member 24 is formed substantially at a center position (left side in FIG. 1) in the axial direction A of the side portion. The outlet-side through hole 22D has a greater length in the axial direction A than that of the outlet-side communication hole 24D. Specifically, as shown in FIGS. 2( a) and 2(b), an upper end of the outlet-side communication hole 24D matches with an upper end of the outlet-side through hole 22D in the state where the movable member 24 is positioned closest to the bottom portion 22A of the housing 22 in the axial direction A (hereinafter, this position is referred to as a first position). Further, as shown in FIGS. 3( a) and 3(b), a lower end of the outlet-side communication hole 24D matches with a lower end of the outlet-side through hole 220 in the state where the movable member 24 is positioned closest to the fixation member 26 in the axial direction A (hereinafter, this position is referred to as a second position).

As shown in FIG. 1, a lower surface of an end 24B of the movable member 24, an inner peripheral surface of the end 22B of the housing 22, and an upper surface of the large diameter portion 26A and a peripheral surface of the small diameter portion 26B of the fixation member 26 form an annular pressure chamber 23E. The length in the axial direction A of the pressure chamber 23E is smaller than that of the accommodation chamber 23. An introducing hole 22E for connecting the pressure chamber 23E with the outside of the housing 22 is formed at the end 22B of the housing 22 (right side in FIG. 1).

The introducing passage 28 is connected to the introducing hole 22E at one end and is connected to an upstream side of the relief valve 21 in the relief passage 16 at the other end.

The switching valve 29 is an electromagnetic solenoid valve and is configured to be opened by energization of the solenoid and closed by stopping of energization of the solenoid.

Output signals from various sensors for sensing an engine operation state are sent to an electronic control device 30 for performing such energization control of the switching valve 29. The sensors include an engine speed sensor 41 for detecting an engine speed NE, an intake air amount sensor 42 for detecting a intake air amount GA, and a water temperature sensor 43 for detecting an engine cooling water temperature THW. A pressure sensor 44 for detecting an oil pressure Ps adjusted by the relief valve 21 is provided. The pressure sensor 44 is provided downstream of the oil pump 14 and downstream of a connection point of the pressure sensor 44 and the relief passage 16 in the main supply passage 11. Further, in this embodiment, a liquid level sensor 45 for detecting a liquid level L of the oil is also provided in the oil pan 12. In this embodiment, a float-type liquid level sensor that outputs a signal corresponding to the oil liquid level L is provided as the liquid level sensor 45. The electronic control device 30 senses the engine operation state on the basis of the output signal from the various sensors and performs energization control of the switching valve 29 according to the engine operation state. Then, by changing the valve opening pressure of the relief valve 21 as described below, the pressure stage of the oil fed to the individual parts of the internal combustion engine is switched between the two stages, the high-pressure stage and the low-pressure stage. The electronic control device 30 corresponds to a control portion according to the present invention.

Next, with reference to FIGS. 2 and 3, operation of the switching mechanism 20 will be described.

When the switching valve 29 is in the opened state, as shown in FIG. 2( a), some of the oil ejected from the oil pump 14 is fed to the pressure chamber 23E through the introducing passage 28 and the introducing hole 22E. Thus, the movable member 24 is pushed upward by the pressure of the oil in the closing direction of the valve body 25 (upward in FIG. 2( a)) and is held at the above-mentioned first position. In this state, when the pressure of the oil ejected from the oil pump 14 is relatively low, the valve body 25 blocks communication between an inlet portion 23C and an outlet portion 23D of the accommodation chamber 23. Thus, the oil ejected from the oil pump 14 is fed to the individual parts of the internal combustion engine without being released to the upstream side of the oil pump 14.

When the pressure of the oil ejected from the oil pump 14 increases with an increase in the engine speed NE from the above-mentioned state, the oil pressure applied to the valve body 25 in the valve opening direction (downward in FIG. 2( a)) also increases, thereby pushing the valve body 25 downward in the valve opening direction. Then, when the valve body 25 is pushed down to the position shown in FIG. 2( b) or a lower position, the inlet portion 23C and the outlet portion 23D of the accommodation chamber 23 are connected with each other. Thus, some of the oil ejected from the oil pump 14 is released to the upstream side of the oil pump 14 through the relief passage 16. As a result, the pressure stage of the oil is set to the low-pressure stage.

Next, when the switching valve 29 is closed, none of the oil ejected from the oil pump 14 is fed to the pressure chamber 23E through the introducing passage 28 and the introducing hole 22E as shown in FIG. 3( a), and the oil existing in the pressure chamber 23E is discharged to the outside of the housing 22 through a discharge passage (not shown). Then, when the oil pressure that pushes the movable member 24 upward decreases, the movable member 24 is pushed downward in the direction of opening the valve body 25 (downward in FIG. 3( a)) and is held at the second position.

When the oil pressure applied to the valve body 25 in the valve opening direction (downward in FIG. 3( a)) increases in the above-mentioned state, and the valve body 25 is pushed downward in the valve opening direction to the position shown in FIG. 3( b) or a lower position, the inlet portion 23C and the outlet portion 23D of the accommodation chamber 23 are connected with each other. Thus, some of the oil ejected from the oil pump 14 is released to the upstream side of the oil pump 14 through the relief passage 16. As a result, the pressure stage of the oil is set to the high-pressure stage.

Next, with reference to FIG. 4, energization control of the switching valve 29 (switching control of the pressure stage of the oil) will be described. FIG. 4 is a flowchart showing a process of energization control of the switching valve 29. A process shown in this flowchart is repeatedly performed through the electronic control device 30 every predetermined period during operation of the internal combustion engine.

As shown in FIG. 4, in the process, first, the current engine speed NE and the current engine load KL are read (Step S101). Next, with reference to a map in FIG. 5, a target pressure stage as a target value of the pressure stage of the oil is set based on the engine speed NE and the engine load KL (Step S102).

As shown in FIG. 5, in the engine operation region where the engine speed NE is low and the engine load KL is low, the target pressure stage is set to the low-pressure stage. In the engine operation region where the engine speed is high and the engine load KL is high, the target pressure stage is set to the high-pressure stage.

Then, as shown in FIG. 4, in Step S103, when the set target pressure stage is the high-pressure stage, energization of the solenoid of the switching valve 29 is stopped so as to close the switching valve 29 (Step S104), and the process is suspended.

In contrast, in Step S103, when the set target pressure stage is the low-pressure stage, energization of the solenoid of the switching valve 29 is performed so as to open the switching valve 29 (Step S105), and the process is suspended.

Next, with reference to FIG. 6, an example of changes of the pressure Ps of the oil with a change in the engine speed NE will be described.

As shown in FIG. 6, when the switching valve 29 is in the opened state, until the engine speed NE increases to N0, the oil pressure applied to the valve body 25 in the valve opening direction is low and the relief valve 21 is in the closed state. Therefore, the pressure Ps of the oil increases with the increase in the engine speed NE.

Then, when the engine speed NE reaches N0 and the oil pressure Ps reaches P0, the oil pressure applied to the valve body 25 in the valve opening direction reaches the valve opening pressure corresponding to the low-pressure stage. As a result, the relief valve 21 is opened, which releases some of the oil ejected from the oil pump 14 to the upstream side of the oil pump 14. Thus, in a region where the engine speed NE is N0 or larger, the degree of the increase in the pressure Ps of the oil with respect to the increase in the engine speed NE is relatively small as compared to a region where the engine speed NE is less than N0, and the pressure stage of the oil is the low-pressure stage.

Then, when the engine speed NE further increases to N1 and the oil pressure Ps reaches P1, the switching valve 29 is closed to switch the pressure stage of the oil to the high-pressure stage. As a result, the movable member 24 is pushed downward, so that the relief valve 21 is closed again. Thus, until the oil pressure applied to the valve body 25 in the valve opening direction reaches the valve opening pressure corresponding to the high-pressure stage (until the engine speed NE reaches N2), the oil pressure Ps rapidly increases. Then, when the engine speed NE reaches N2 and the oil pressure Ps reaches P2, the relief valve 21 is opened, thereby releasing some of the oil ejected from the oil pump 14 to the upstream side of the oil pump 14. Thus, in a region where the engine speed NE is N2 or larger, the degree of the increase in the pressure Ps of the oil with respect to the increase in the engine speed NE is relatively small and the pressure stage of the oil is the high-pressure stage, as compared to a region where the engine speed NE is N1 or larger and is less than N2.

In such a control device for an internal combustion engine, for example, when the pressure Ps of the oil fed to the individual parts of the engine does not need to increase much, the mechanical load of the internal combustion engine due to driving of the oil pump can be reduced by setting the pressure stage of the oil to the low-pressure stage, which improves fuel efficiency. When the pressure Ps of the oil fed to the individual parts of the internal combustion engine needs to be high, by changing the pressure stage of the oil to the high-pressure stage, the reliability of the engine is ensured.

The oil liquid level L is determined as abnormal through the electronic control device 30 on the basis of the fact that the oil liquid level L detected by the liquid level sensor 45 falls below a predetermined value Lth. In this embodiment, in the case where the pressure stage of the oil is set to the low-pressure stage through the switching mechanism 20, the predetermined value Lth in the liquid level determination is set to a larger value as compared to the case where the pressure stage of the oil is set to the high-pressure stage. Thereby, abnormality of the oil liquid level L can be correctly determined. The electronic control device 30 corresponds to a setting portion according to the present invention.

Next, with reference to FIG. 7, the liquid level determination will be described. FIG. 7 is a flowchart showing a processing procedure of the liquid level determination. The process shown in this flowchart is repeatedly performed through the electronic control device 30 every predetermined period during operation of the internal combustion engine.

As shown in FIG. 7, in the process, first, it is determined whether or not an execution condition of the liquid level determination is satisfied (Step S201). It takes a predetermined time, after switching of the pressure stage of the oil, for the liquid level L of the oil in the oil pan 12 to become the level corresponding to the switched pressure stage pressure. Thus, when the oil liquid level L is determined on the basis of the predetermined value set according to the switched pressure stage, for example, immediately after switching of the pressure stage of the oil, false determination can occur. Thus, in this embodiment, a state where at least a predetermined period Δt1 has passed since switching of the pressure stage of the oil is set as the execution condition of the liquid level determination. When it is determined that the execution condition of the liquid level determination is not satisfied (Step S201: NO), the process is suspended.

In contrast, in Step S201, when it is determined that the execution condition of the liquid level determination is satisfied (Step S201: YES), it is determined whether or not the pressure stage of the oil at this time is the high-pressure stage (Step S202). As a result, when it is determined that the pressure stage of the oil is the high-pressure stage, a high-pressure stage predetermined value LthHI (NE) is set as the predetermined value Lth (Step S203). In contrast, in Step S302, when it is determined that the pressure stage of the oil is the low-pressure stage, a low-pressure stage predetermined value LthLO (NE) is set as the predetermined value Lth (Step S204).

As shown in FIG. 8, at the same engine speed NE, the low-pressure stage predetermined value LthLO (NE) is set to be larger than the high-pressure stage predetermined value LthHI (NE). Since the individual parts of the internal combustion engine require more oil as the engine speed NE is higher, the predetermined values LthHI (NE), LthLO (NE) are set to increase respectively as the engine speed NE increases. The predetermined values LthHI (NE), LthLO (NE) are optimum values obtained through an experiment or the like.

When the predetermined value Lth is set in this manner, it is determined whether or not the liquid level L detected by the liquid level sensor 45 is smaller than the predetermined value Lth (Step S205). As a result, when it is determined that the liquid level L is not smaller than the predetermined value Lth (Step S205: NO), it is determined that the oil liquid level L is not abnormal (Step S206) and the process is suspended.

In contrast, in Step S205,when it is determined that the liquid level L is smaller than the predetermined value Lth (Step S205: YES), it is determined that the oil liquid level L is abnormal (Step S207) and the process is suspended.

The control device for an internal combustion engine in the above-described embodiment can achieve the following advantages.

(1) The electronic control device 30 is used to control the actions of the switching mechanism 20 for switching the pressure stage of the oil, which is fed from the oil pan 12 to the individual parts of the internal combustion engine, between the two stages, or the low-pressure stage and the high-pressure stage. Further, the liquid level determination is performed for determining that the oil liquid level L is abnormal on the basis of the fact that the liquid level L of the oil in the oil pan 12, which is detected by the liquid level sensor 45, falls below the predetermined value Lth. In the case where the pressure stage of the oil is set to the low-pressure stage through the switching mechanism 20, the predetermined value Lth in the liquid level determination is set to a larger value, as compared to the case where the pressure stage of the oil is set to the high-pressure stage. Thus, the predetermined value Lth used for comparison with the oil liquid level L in the liquid level determination is not set to a common value irrespective of the pressure stage of the oil, but is variably set on the basis of relationship between the pressure stage of the oil and the oil liquid level, that is, the relationship that in the case where the pressure stage of the oil is set to the low-pressure stage, the oil liquid level L is higher, as compared to the case where the pressure stage of the oil is set to the high-pressure stage. Therefore, in the internal combustion engine including the switching mechanism 20 for switching the pressure stage of the oil between the two stages, or the low-pressure stage and the high-pressure stage, abnormality of the oil liquid level L can be correctly determined.

(2) The liquid level determination is prohibited until the predetermined period Δt1 has passed since switching of the pressure stage of the oil. Thereby, it is possible to prevent the liquid level determination from being executed before the liquid level L of the oil in the oil pan 12 becomes the level corresponding to the switched pressure stage since switching of the pressure stage of the oil. Therefore, it is possible to prevent false determination from being made due to the liquid level determination made immediately after switching of the pressure stage of the oil.

The control device for an internal combustion engine according to the present invention is not limited to the configuration exemplified as the above-mentioned embodiment and may be appropriately modified to the following structures, for example.

Although the switching mechanism 20 for switching the pressure of the oil, which is fed to the individual parts of the internal combustion engine, between the two stages, the low-pressure stage and the high-pressure stage, is used in the above-mentioned embodiment, the pressure stage of the oil may be switched among three or more stages.

Although the switching mechanism including the relief valve for releasing some of the oil ejected from the fixed displacement oil pump 14 and the switching valve as a change portion for changing the valve opening pressure of the relief valve is used in the above-mentioned embodiment, the configuration of the switching mechanism is not limited to this. For example, a variable displacement oil pump capable of varying the pressure of the oil ejected from the oil pump may form the movable mechanism.

In the above-mentioned embodiment, in consideration of the fact that it takes the predetermined time, after switching of the pressure stage of the oil, for the liquid level L of the oil in the oil pan 12 to become the level corresponding to the switched pressure stage pressure, the liquid level determination is prohibited until the predetermined period Δt1 has passed since switching of the pressure stage of the oil. However, the execution mode of the liquid level determination is not limited to this, and the determination portion for making the liquid level determination may determine that the oil liquid level L is abnormal on the basis of the fact that the oil liquid level L detected by the liquid level sensor 45 remains below the predetermined value Lth for a predetermined period. In this case, even if the oil liquid level L temporarily falls below the predetermined value Lth before the liquid level L of the oil in the oil pan 12 becomes the level corresponding to the switched pressure stage since switching of the pressure stage of the oil, the oil liquid level L is not determined to be abnormal when duration of such state is less than the predetermined period. Therefore, it is possible to prevent false determinations from being made due to the oil liquid level L falling below the predetermined value Lth immediately after switching of the pressure stage of the oil. 

1. A control device for an internal combustion engine, the engine including a switching mechanism for switching a pressure stage of oil, which is fed from a storage portion to individual parts of the engine, between a plurality of stages, the control device comprising: a control portion for controlling action of the switching mechanism; a liquid level sensor for detecting the liquid level of oil in the storage portion; a determination portion for determining that the liquid level of the oil is abnormal based on the fact that the oil liquid level detected by the liquid level sensor falls below a predetermined value; and a setting portion for setting the predetermined value to a larger value if the pressure stage of the oil is set to a low-pressure stage through the switching mechanism, as compared to a case where the pressure stage of the oil is set to a high-pressure stage.
 2. The control device for an internal combustion engine according to claim 1, wherein the determination portion prohibits determination until a predetermined period has passed after the control portion switches the pressure stage of the oil.
 3. The control device for an internal combustion engine according to claim 1, wherein the determination portion determines that the oil liquid level is abnormal based on the fact that the oil liquid level detected by the liquid level sensor remains below the predetermined value for a predetermined period.
 4. The control device for an internal combustion engine according to claim 1, wherein the switching mechanism comprises a relief valve for releasing some of oil ejected from an oil pump driven by the internal combustion engine to an upstream side of the oil pump and a change portion for changing a valve opening pressure of the relief valve.
 5. The control device for an internal combustion engine according to claim 1, wherein the switching mechanism switches the pressure stage of the oil between two stages, which are a low-pressure stage and a high-pressure stage. 